Differential pressure sensors measure a difference in pressure between two points of measurement (e.g. P1 and P2) of a fluid. A differential pressure sensor (or transducer) converts the pressure difference to an electrical signal that can be measured to determine the differential pressure. For example, a differential pressure sensor may be used in an oil pipe to measure the pressure before and after an orifice in the oil pipe, from which the flow rate of the oil can be determined. Such devices are typically manufactured using micro-machined or Micro-Electro-Mechanical System (MEMS) based techniques. One common technique for manufacturing a pressure sensor is to attach a MEMS device onto a substrate, such as a ceramic or printed circuit board (PCB) substrate, along with etching and bonding techniques to fabricate very small, inexpensive devices.
The pressure-sensing die may typically be formed from a semiconductor material such as silicon. FIG. 1 is a sectional view of a MEMS type pressure sensing die 100 of the prior art. The die 100 maybe formed from a silicon wafer by methods such as dicing to produce a silicon structure 101. The structure 101 is thinned to create a cavity 105 and a thinned portion defining a diaphragm 103. The semiconductor structure 101 may be thinned by any suitable means. For example, the structure 101 may be thinned using anisotropic etching as known in the art. Resistive elements are formed on the surface of the diaphragm 103. The resistive elements exhibit resistance that is proportional to the strain placed on the thinned semiconductor material forming the diaphragm 103.
FIG. 2 is an illustration of a prior art differential MEMS pressure sensor using pressure sensing die/device 100. Pressure sensing device 100 may be mounted to a support structure 207 which is, in turn bonded to a base plate 201, which may be formed from a non-corroding material, for example, stainless steel. The sensing die 100 and the support structure 207 may be bonded to base plate 201, which may also be termed a header, by an adhesive 205. The support structure 207 is used as it isolates the pressure sensing device 100 from sources of strain that are unrelated to pressure, such as thermal expansion which varies between the pressure sensing device 100 and the base plate 201. An opening 203 is defined in the base plate 201 defining an aperture which is in gas or fluid communication with the underside of the diaphragm of pressure sensing device 100. The opening 203 is in communication with a first gas or fluid whose pressure PI is to be measured and which comes in contact with one side of the pressure sensing device 100. The pressure sensing die 100 is attached to the base plate 201 over the opening 203 via support structure 207. Support structure 207 may be formed from glass or similar material which has a coefficient of thermal expansion closer to that of the silicon pressure sensing die 100 as compared to the coefficient of thermal expansion of the stainless steel making up the base plate 201. This matching of the coefficients of thermal expansion prevents exertion of forces on the die 100 not related to pressure, but rather, caused by the strain related to the dissimilar rates of expansion between the die 100 and the base plate 201. The constraint 207 is attached to the base plate 201 by an appropriate adhesive 205 as known in the art. For example, bonding may be performed by a Silicone adhesive, epoxy, solder, braze or other commonly known techniques.
The pressure sensing device 200 includes upper housing 223. Upper housing 223 is configured to provide a sealed attachment to base plate 201. An enclosed volume is defined between upper housing 223 and base plate 201. Flexible corrugated diaphragm 221 serves to divide the enclosed volume into a first volume 219 and a second volume 227. Port 225 is defined through a wall of upper housing 223 and is in communication with a second section or portion of gas or fluid whose pressure P2 is to be measured, and which comes in contact with another side of the pressure sensing device 100. Port 225 may be coupled to a fluid source or gas source which is to be tested for pressure. Pressure sensing die 100 further includes electrical components which create and transmit an electrical signal indicative of a pressure exerted on the die 100. In applications where the fluid being tested is a harsh medium, such as fuel or oil, such media may corrode the electrical components of the die 100. In such embodiments, isolation of the die 100 from the fluid being tested may be accomplished by flexible corrugated diaphragm 221. An oil fill port 215 is provided through the base plate 201. The oil fill port allows the volume 219 between the die 100 and the diaphragm 221 to be filled with a non-corrosive fluid such as silicone oil. When the cavity defining volume 219 is filled, the oil fill port 215 is sealed, for example, by welding a ball 217 across the opening of the oil fill port 215. The oil in volume 219 is thus fully enclosed and in fluid communication with the upper surface of die 100.
Port 225 may be threaded to allow the pressure sensing device 200 to be attached to a line or other transmission means in communication with the gas or fluid to be tested or measured. The gas or fluid being measured enters the port 225 and fills the interior volume 227. When the interior volume 227 is filled, the fluid being measured is in contact with the upper side of the flexible diaphragm 221. Pressure exerted by the gas or fluid being measured is transmitted through the flexible diaphragm 221 to the enclosed volume 219 of oil. The force applied to the oil by the flexible diaphragm 221 is transmitted throughout the oil and to the surfaces containing the oil, including the upper surface of pressure sensing die 100.
When pressures P1 and P2 are exerted on pressure sensing die 100, an electrical signal through piezo-resistive elements formed in the upper surface of the diaphragm of pressure sensing die 100 varies responsive to variations in the piezo-resistive elements. The electrical signal is representative of the differential force applied to the surface of the pressure sensing die 100. The electrical signal is conducted via bond wires 209 to conductive pins 211 which may be electrically connected to other system circuitry, such as a control circuit, or converted to pressure data which may be stored, by way of non-limiting example, in an electronic memory.
The flexible diaphragm 221 and oil filled volume 219 isolate the die 100, bond wires 209 and conductive pins 211 from the corrosive or harsh media being measured via port 225. Additionally, the volume 219 containing the oil must be sealed such that leakage or contamination of the oil within volume 219 does not occur. Conductive pins 211 carrying the electrical signal from the pressure sensing die 100 must pass through the base plate 201 to allow external connection of other system components. Conductive pins 211 are enclosed in a glass or ceramic material fired into a tube or opening 213 which forms a hermetic seal with base plate 201. Hermetic seals are expensive to produce and fragile, but are necessary to ensure the integrity of the volume 219. A pressure sensor which provides isolation of the sensing components and associated circuitry from harsh media being measured in a simple and inexpensive form factor is therefore desired.
Differential pressure sensors may be used in high pressure applications in which the measured Psi in the two different portions are as high or higher than 1000 psi. For example a differential pressure sensor may measure the difference in pressure across an orifice in a high pressure, flowing fluid or gas application. In such an application, the pressure may be 1001 psi on one side of the orifice and the pressure on the other side may be 1000 psi. At high pressures, a differential pressure sensor may exhibit common mode (or line pressure) errors, which may be caused by strain that is unrelated to the pressure, such as thermal expansion which varies between the different materials used in the pressure sensing device 100. While the differential pressure sensors are supposed to only measure the difference in pressure P1 vs. P2, often it is difficult, especially when dealing with high pressure applications, to discern differential pressure from the line pressure or “common mode” pressure. For example, in a situation where there is no differential pressure and no common mode errors, the differential pressure sensor will read zero pressure. However when the pressures P1 and P2 are both 1000 psi, prior art pressure sensors may exhibit common mode error which results in an inaccurate differential pressure of 1 psi.
Methods to eliminate the common mode error are difficult. A novel apparatus and method to eliminate common mode error in differential pressure sensors is therefore desired.